WHAT WE KNOW ABOUT TITAN
With its 5150-km diameter, Titan is the largest moon of Saturn and the 2nd-largest planetary satellite in Milky Way after Ganymede (5276 km). But, it is the only natural satellite known to have a dense atmosphere with clear evidence of stable bodies of surface liquid. Its atmosphere is largely nitrogen with clouds of methane and ethane. The climate is dominated by seasonal weather patterns as on Earth with its rain and wind, which creates some dunes, rivers, lakes, seas and deltas.
Because its thick atmosphere, Titan’s pressure and density at the surface is greater than that on Earth, which it is at approximately 8 km altitude similar to our one.
The atmospheric temperature on Titan decreases fairly linearly from the surface up to approximately 60 km in altitude.
Outside the atmosphere, the solar intensity is only approximately 15 W/m2. And so, when coupled with the haze and clouds within the atmosphere that make the surface light very weak.
Because the relative humidity of methane on Titan is only ~50 percent, providing a thermodynamic out of balance, a body of pure methane cannot persist indefinitely on its surface.
We know that by the use of terrestrial empirical transfer coefficients who indicate us that the evaporation rate has been estimated at up to 1 m/yr. And, because of its strong link with the wind speed. The saturation vapour pressure of ethane is very low, as we see.
Titan Atmospheric Composition
Ethane suppresses the partial pressure of methane above mixed-composition seas, that has proved a complex Titan’s air-seas interactions. For now, we suppose that ethane probably migrates only over long periods, and so evaporation and precipitation of methane may be much more like terrestrial weather.
In fact, transient surface darkening has been observed at low latitudes on Titan in association with methane clouds, followed by brightening, suggesting that shallow flooding occurred, followed by evaporation.
The winds within the atmosphere blow fairly consistently in the same direction as the planetary rotation. At altitudes that can be considered for airship operation, below 20 km, the wind speed decreases from approximately 5 m/s to near 0 at the surface.
The gravitational acceleration on Titan (1.35 m/s2) is less than that of Earth’s moon. Although the atmosphere is composed mostly of nitrogen, the speed of sound within the atmosphere is about half that on Earth. This is mainly due to the low temperature of the atmosphere compared to Earth’s.
Density, temperature, and wind velocity of the atmosphere are critical in determining the feasibility of airship flight on Titan.
The Cassini’s radar discovers in 2006, when the Titan was in winter darkness, seas in the North Polar Region. And, after having more or less fully-mapped these seas in 2013, attention was drawn to exploration of liquid environments on Titan.
In order of ascending size, they are Punga Mare, Ligeia Mare, and Kraken Mare
These changes are particularly strong at the winter pole. Among the gases produced by photochemistry is ethane, which is a liquid at Titan conditions, and is expected to accumulate on the surface.
In Northern Winter Darkness position, Cassini radar observations in 2006, bodies of standing liquid were confirmed. Hundreds of radar-dark lakes, typically 20 km across, were discovered at about 70° N. Latitude.
As see in the picture below, the first sea to be observed was Ligeia Mare, a 300-400 km wide body, following by the smaller Punga Mare, who is closer to the North Pole, and the giant Kraken Mare sprawls over 1,000 km towards mid-latitudes. The three big seas were named by International Convention.
In the period of 2004-2010, Cassini were placed in the Southern hemisphere, where the place was better illuminated. It has discovered a body liquid of 70 by 250 km, later named Ontario Lacus. The Orbiter confirmed presence of ethane on it.
Further analysis of the near-IR data suggests that Ontario Lacus may in fact be muddy, and a bright margin is suggestive of a ‘bathtub ring’ of evaporate deposits.
Here, these are not solubility salts in terrestrial waters as Earth, but sure some organic analog has been probably deposited at the shrinking margins. In fact, some years later, a comparison between an optically measured outline and the margins in a radar image suggested that Ontario may have shrunk in extent due to seasonal evaporation and the very shallow regional slopes. So, Ontario lacus is most likely only a few meters deep.
Three seas have been mapped by Voyager & Cassini: Punga, Kraken, and Ligeia Mare, all in north polar region, all with ethane/methane/nitrogen compositions